Pedagogical tasks in language learning settings promote learner-learner interaction and provide second language (L2) learners with opportunities to process authentic input and produce output (Philp, Adams, & Iwashita, 2014). During these interactions, learners use their language repertoire to mediate their output (Swain & Lapkin, 2000). Although language mediation has been investigated in various L2 settings, few studies have explored the mediating functions of the languages of plurilingual learners from a sociocultural perspective. The current case study examined the specific mediating functions of language sources (i.e., first, second, and third language) in the oral and written modalities during different types of pedagogical tasks among four students learning French as a third language (L3) in Mexico. Data sources included interactional turns produced during task completion and one-on-one interviews. The findings indicated that task modalities and task types impact the specific mediating functions of the L1 and L3. In addition, the quantity of L1 mediation appeared to be impacted by the learners' linguistic proficiencies. Implications for foreign language pedagogy are presented.